HF links provide an out-of-line-of-sight capability that makes it possible to carry out communications at long or even very long distance without requiring a satellite to relay the transmission.
The radio waves of which the frequency is in the HF band, also called decametric waves with reference to their wavelength which is between 10 and 100 meters, are most frequently propagated according to two alternative modes.
In a first propagation mode, called the ionospheric mode, the HF radio waves sustain a reflection on the various layers of the ionosphere. Since these layers are not stable over time or in space, they cause considerable variations of the propagation channel which causes instabilities and hence a reduction of the available bit rate. The capability of HF links being propagated in an ionospheric mode is thus limited in terms of available transmission bit rate.
In a second propagation mode, called the surface mode, the HF radio waves are propagated as ground waves. The maximum propagation distance then depends greatly on the composition of the land surface over the path between the transmitter and the receiver. Specifically, a very conductive surface, such as salt water for example, provides a much greater range than a ground of the rocky type for example. Although the surface types vary much less over time than the ionospheric layers, the propagation of the HF radio waves in surface mode remains extremely variable from one location to another on the globe. This aspect has an impact notably in the case of mobility of the transmitter or the receiver.
The frequencies available in the HF band are allocated to the various users by the International Telecommunications Union (ITU). A channeling that is conventionally used by HF communication systems in ground wave or in ionospheric wave is of the order of 3 kHz optionally doubled to 6 kHz. The modulation mode currently used is that of the Single Side Band or SSB. This channeling is imposed by the standards defining the HF waveforms.
The bandwidth limitation imposed by 3 kHz channeling poses the problem of limiting the payload bit rate that can be envisaged for an HF transmission. Because of the small bandwidth, but also the limitations induced by the ionospheric propagation channel or in surface wave, the bit rates achieved do not exceed approximately ten kilobits per second. As an example, the typical maximum value of the current standards is 9600 bits per second for 3 kHz of bandwidth. For applications requiring greater resources, such as Internet or videophone applications for example, the payload bit rate proposed by the existing HF waveforms is insufficient.
Moreover, the frequency bands allocated to one and the same user are not usually contiguous and are spread throughout the whole HF band according to an imposed frequency plan.
It therefore becomes a problem to increase the payload bit rate while complying with the specific constraints of HF communications and particularly HF communications in ionospheric mode.
A known solution for tackling the instabilities of the HF propagation channel consists in carrying out an automatic search for an available frequency amongst those allocated to the user. This procedure is carried out initially or after communication breakdown of a link, but does not make it possible to take account subtly and dynamically of the temporal evolution of availability of the medium.
Increasing the payload bit rate is conventionally envisaged according to two methods. A first isoband solution consists in retaining the imposed channeling and in increasing the spectral efficiency of the modulation and/or the efficiency of the correction coding protecting the communication against the instabilities of the channel. This solution has a theoretical limit in achievable bit rate imposed by the channeling and moreover the increase in efficiency of the correction coding adversely affects the link by reducing the range and the probability of establishment of the communication.
A second solution consists in increasing the channeling width by taking account of the adjacent channels. This solution has several drawbacks. First of all, the complexity of the systems, transmitters and receivers, is increased with the increase in the payload frequency band. For the transmitter, the production of the broadband-forming filter and above all that of the power amplifier and of the frequency synthesizer becomes more complex as the frequency band increases. For the receiver, it is the production of the equalizer that is made more complex with the increase in the payload band. Furthermore, the contiguous channels are most frequently not those that have the best physical availability and may be affected by the disruptions already mentioned that are associated with the ionospheric layer. Specifically, since the HF medium fluctuates greatly, it is not very likely to have a large number of contiguous channels that are physically available and that would make it possible to obtain the envisaged payload bit rates. Finally, from a legal point of view, the frequency bands allocated to a user by the ITU cannot be easily modified to exploit several contiguous bands. On the contrary, the allocated frequency plan most frequently corresponds to a set of frequencies distributed throughout the whole of the HF band so as to be able to make the transmissions secure by the application of methods of frequency-hopping or of limiting the probability of using a frequency that might be disrupted.